This invention relates to pentagonal prisms and related optical systems for single lens reflex cameras.
Recently, single lens reflex cameras are being equipped with a variety of auxiliary optical systems operable in association with a finder optical system including a pentagonal prism. Typical auxiliary optical systems are, for example, an indicating optical system for indicating within a finder view field picture-taking information marked on a lens barrel as well as picture-taking information provided within the camera body; a light measuring optical system for directing light transmitted through an objective lens to a light receiving element; and a stray light eliminating optical system for shutting out ghosts or light which is inversely incident through an ocular lens.
It has been customary to use a holding member for attaching such an auxiliary optical system to a camera body. Such structure has the following disadvantages:
(I) The use of a holding member for supporting an individual optical system results in increased manufacturing costs and requires a large space.
(II) Foreign matter, such as dust, is admitted into the auxiliary optical system or between the pentagonal prism and the auxiliary optical system.
(III) It is difficult to make adjustment of the relative position between the pentagonal prism and the auxiliary optical system.
Meanwhile, it has been proposed that an individual element in an auxiliary optical system, for example, a prism, be molded of plastic. This method presents another difficulty in that, because the individual element to be molded is small, a smooth molten plastic flow cannot be achieved upon molding, resulting in failure to produce an element of high precision and uniform in quality.
In the vicinity of the pentagonal prism, on the other hand, are provided optical elements, such as a condenser lens and an ocular lens, constituting a finder optical system, besides the auxiliary optical system. The same difficulties as in the case of the auxiliary optical system are also experienced in supporting these optical elements and in the positional adjustment of these elements relative to the pentagonal prism.
Japanese Utility Model Application No. 46-76297, which was laid open to public inspection on Apr. 21, 1973 discloses a single lens reflex camera having a device for indicating photographic information in a viewfinder, which device may avoid the above described disadvantages (I) and (II). More particularly, in the device, the frame on which a pentagonal prism is mounted is made of a transparent plastic resin and includes a portion which serves as an optical element to introduce light bearing photographic information or light from an indicating lamp to the pentagonal prism through the lower light incident surface thereof. This construction requires no means for holding the optical element and prevents foreign matter, such as dust, from entering between the optical element and the pentagonal prism. However, since the frame is so adapted that the pentagonal prism is only mounted thereon, i.e., the frame has no means for holding the pentagonal prism in the horizontal direction, adjustment is required to determine the relative position of the optical element with respect to the pentagonal prism in the horizontal direction, and thus the device will still have disadvantage (III).